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Fine-root traits mediate the capacity of plants to acquire soil resources in different environments. This study aimed to examine the changes of fine-root traits when roots proliferate into the litter layer vs. mineral soils, and to determine fine-root trait plasticity of these roots in response to nitrogen (N) addition. A one-year N addition experiment was conducted in a 22-year-old broadleaf Mytilaria laosensis (Hamamelidaceae) plantation in subtropical China. Newly produced fine roots were collected monthly from the litter layer and upper mineral soil (0–10 cm) layer to measure root morphological traits and nutrient concentrations. Fine-root production was determined using ingrowth mesh screens in the litter layer. Fine-root production in the litter layer in the Mytilaria laosensis plantation was 2.6 g m−2 yr.−1 but increased 3- to 5-fold with N addition. Significant differences in fine-root morphological traits and nutrient concentrations were found between the litter layer and 0–10 cm mineral soil layer. Fine roots in the litter layer were thinner, with higher specific root length (SRL), higher specific root area (SRA), a higher proportion of fine-root biomass in lower, more absorptive root orders, and lower root tissue density (RTD) than those in 0–10 cm mineral soil layer. Higher carbon (C), N, phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) concentrations and lower C:N ratio (C/N) were also observed in fine roots in the litter layer, compared to the 0–10 cm mineral soil layer. Nitrogen addition significantly increased root P, K, and Ca concentrations, but had no effect on Mg concentration. Nitrogen addition did not affect most fine-root morphological traits but did result in decreased root diameter. Compared with the mineral soil, roots produced in the litter layer generally reflected a more absorptive strategy with smaller root diameter and lower RTD and with higher SRL, SRA, and nutrient concentrations which together are generally associated with more metabolically active, but shorter lived roots. Strong responses of fine-root production and nutrient concentrations to N addition also suggest that N may be a driving factor for fine-root growth into the litter layer. Further studies are required to identify the effect of fine-root growth into the litter layer on microbial activity.

This study aimed to determine the influence of tree species on soil microbial community structure. We conducted a litter and root manipulation and a short-term nitrogen (N) addition experiment in 19-year-old broadleaf Mytilaria laosensis (Hamamelidaceae) and coniferous Chinese fir (Cunninghamia lanceolata) plantations in subtropical China. Phospholipid fatty acid (PLFA) analysis was used to examine treatment effects on soil microbial community structure. Redundancy analysis (RDA) was performed to determine the relationships between individual PLFAs and soil properties (soil pH, carbon (C) and N concentration and C:N ratio). Soil C:N ratio was significantly greater in M. laosensis (17.9) than in C. lanceolata (16.2). Soil C:N ratio was the key factor affecting the soil microbial community regardless of tree species and the litter, root and N treatments at our study site. The fungal biomarkers, 18:1ω9 and 18:2ω6,9 were significantly and positively related to soil C:N ratio and the abundance of bacterial lipid biomarkers was negatively related to soil C:N ratio. N addition for 8 months did not change the biomass and structure of the microbial community in M. laosensis and C. lanceolata soils. Soil nutrient availability before N addition was an important factor in determining the effect of N fertilization on soil microbial biomass and activity. PLFA analysis showed that root exclusion significantly decreased the abundance of the fungal biomarkers and increased the abundance of the Gram-positive bacteria. Rootless plots had a relatively lower Gram-positive to Gram-negative bacteria ratio and a higher fungi to bacteria ratio compared to the plots with roots under both M. laosensis and C. lanceolata. The response of arbuscular mycorrhizal fungi (16:1ω5) to root exclusion was species-specific. These observations suggest that soil C:N ratio was an important factor in influencing soil microbial community structure. Further studies are required to confirm the long-term effect of tree species on soil microbial community structure.

The knowledge of individual tree species impacts on soil respiration based on rigorous experimental designs is limited, but is crucial to help guide selection of species for reforestation and carbon (C) management purposes. We assessed monthly soil respiration and its components, litterfall input, fine root production and mortality under 19-year-old native coniferous Cunninghamia lanceolata and broadleaved Mytilaria laosensis plantations in sub-tropical China. Total soil respiration from October 2011 to March 2013 was significantly lower under the C. lanceolata than the M. laosensis plantation. The difference in respiration rates derived from fine roots and the litter layer explained much of the variation of total soil respiration between the two tree species. We used an exponential equation and base temperature (10 °C) to normalize soil respiration rate and its components (R10) and determined the correlation between R10 and soil moisture. Although soil moisture had a positive relationship with R10 derived from roots or litter under both C. lanceolata and M. laosensis forests, these positive correlations were masked by negative relationships between soil moisture and R10 derived from root-free soil, which resulted in a neutral correlation between total R10 and soil moisture under C. lanceolata forests. Monthly litterfall input was associated with variation in concurrent total soil respiration rate under the M. laosensis plantation and respiration rate lagging 3 months behind under the C. lanceolata plantation, which may suggest that litterfall input from M. laosensis can more rapidly produce C substrates for microbial respiration than litterfall from C. lanceolata. This study highlighted that tree species-induced variation in the quality and quantity of fine roots and litterfall can impact not only the soil respiration rate but also the seasonal variation model of forest soil respiration.